Progesterone helps maintain a pregnancy until birth, according to the American Pregnancy Association. The corpus luteum produces the hormone to thicken and prepare the uterine lining for implantation. It continues to produce progesterone until around 10 weeks, when the placenta takes over production.
Progesterone prevents uterine contractions during pregnancy, decreases prostaglandin and prevents the body from having an immune response to the fetus, reports the American Pregnancy Association. A woman may have low levels of this hormone during her pregnancy, which can lead to early loss of the fetus. To prevent this, physicians may prescribe supplemental progestins, explains Mayo Clinic.